1. Field of the Invention
The invention relates to apparatus for combusting preheated fuel and/or preheated oxidant.
2. Related Art
There are three basic types of combustion systems based on temperature of the fuel and oxidant: First and most common burner system utilize unheated (or ambient) fuel and oxidant for combustion. Both air-fuel and oxy-fuel burners of above types are widely used in industry (see U.S. Pat. Nos. 5,199,866; 4,690,635).
The second type of burner system employs preheating the ambient fluids (fuel and oxidant) inside the burner embodiment. This method employs ambient or slightly preheated fuel and oxidant as an input to the burner. It is commonly used with air-fuel burners and combustion engines. U.S. Pat. No. 4,257,762 describes one such method where preheated forced draft air is used for preheating fuel gas by partial mixing in the burner passage. In another application (U.S. Pat. No. 5,413,477), hot flue gas is entrained inside the burner to preheat fuel and combustion air using fuel-rich and fuel-lean staged combustion. On oxy-fuel combustion systems, the concept has been adapted and the preheating of natural gas is used by mixing with another hot fluid, or partial combustion in an oxygen poor atmosphere that leads to soot formation as well as preheating (U.S. Pat. No. 5,725,366). These are known technologies where preheating of either oxidant or fuel is carried out within the burner body or burner block. In summary, the burner or burner block is used as a heater for fuel, oxidant or both. In the first stage, partial combustion of fuel with oxidant is carried out and in the second stage, subsequent mixing of hot combustion products from first stage with the remaining fuel and oxidant is carried out. Thus, overall preheating of fuel and oxidant is achieved.
Preheated air for air-fuel burner systems is known. However, most applications involve preheated combustion air (U.S. Pat. No. 4,492,568; U.S. Pat. No. 5,823,769). The traditional methods employ a refractory heat exchanger (two regenerators) to preheat combustion air in a cyclic manner. Thus, with air-fuel burners and ceramic regenerators, preheating temperatures as high as 1100.degree. C. for air containing 21 (volumetric) percent oxygen is quite common. The air is preheated in such devices by periodic (or cyclic) flow through a given regenerator (such as checkers containing ceramic elements) that have been preheated by the hot flue gases during the previous cycle. The disadvantage of above heat recovery system is that it can not utilize pure oxygen. The first reason is safety related. The flue gas-leaving the furnace is usually dirty due to entrained process particulates, fuel, condensate and vapors, which can deposit on the heated checker surfaces in one cycle and then react readily with preheated oxygen in the next cycle. This may create explosive conditions. The second reason is due to slippage of preheated oxygen (precious commodity) through refractory cracks and joints of the regenerator structure.
The use of metallic recuperators is also widespread but the preheat temperatures are lower than 700.degree. C. due to the metallic construction and corrosion effects of hot oxidant (air) and flue gases on the metallic parts of the recuperator. Yet these kinds of air-fuel heat recovery systems have lower thermal efficiency due to the nitrogen contained in the air. This inert nitrogen has to be heated to process temperature and this heat is simply wasted. In addition, nitrogen at high temperature triggers the forming of NOx.
The preheated oxygen for combustion has been used before in the case of a reforming reactor (U.S. Pat. No. 5,588,974) where oxygen and steam are used to transform hydrocarbons into hydrogen and carbon monoxide. The hot oxidizing mixture is fed into the reactor at temperatures ranging from 500.degree. F. to 1200.degree. F. The object was to reform fuel into H.sub.2 and CO by partial combustion. The combustion was not carried out in stoichiometric proportions to release heat for heating applications such as steel melting, glass melting, heat treatment, etc. The objective of the present invention is different since it deals with a combustion burner, where fuel is combusted with oxygen in nearly stoichiometric proportions.